1. Field of the Invention
This invention is directed to an adhesion promoter layer having a reduced acidity, and, in particular, an adhesion promoting layer containing a binder and a conducting material having a reduced acidity.
This invention is further directed to solid electrolytic cells (batteries) containing an anode, a solid electrolyte, a cathode, and an adhesion promoter layer having a reduced acidity.
2. State of the Art
Electrolytic cells containing an anode, a cathode and a solid, solvent-containing electrolyte incorporating a salt are known in the art and are usually referred to as "solid batteries". These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., "liquid batteries") including improved safety features. Notwithstanding their advantages, the manufacture of these solid batteries requires careful process controls to maximize the adherence of the various layers during formation of the electrolytic cells. Poorly adhered laminates can inhibit battery performance and can significantly reduce charge and discharge capacity.
Typically, a cathode is formed on a current collector, such as a metal foil. On particularly smooth foils, the cathode does not adhere well. A solution to this problem includes roughening the surface of the metal foil. Roughening of the foil can be economically disadvantageous and also contributes an undesirable extra processing step. Another solution is to apply an adhesion promoter layer to the current collector. Adhesion promoter layers made of a binder and a conducting medium, e.g., polyacrylic acid and carbon, cause problems relating to reduction of lithium. The acidity of the binder can react with the lithium metal anode in the battery, reducing tile amount of available lithium within the system and decreasing the potential cycle life of the battery. Additionally, the introduction of H.sup.+ ions into the cathode can occupy potential Li.sup.+ sites, thus reducing the capacity of the battery.
Typically, solid batteries employ a solid electrolyte interposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix and a suitable salt, such as an inorganic ion salt, as a separate component. The inorganic matrix may be non-polymeric, e.g, .beta. alumina, silver oxide, lithium ioctide, and the like, or polymeric, e.g., inorganic (polyphosphazene) polymers, whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic monomers include, by way of example, ethylene oxide, propylene oxide, ethyleneimine, epichlorohydrin, ethylene succinate, and an acryloyl-derivatized alkylene oxide containing an acryloyl group of the formula CH.sub.2 .dbd.CR'C(O)O-- where R' is hydrogen or a lower alkyl of from 1-6 carbon atoms.
Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are generally not preferred and the art typically employs a solid electrolyte containing a polymeric matrix. Nevertheless, electrolytic cells containing a solid electrolyte containing a polymeric matrix suffer from low ion conductivity and, accordingly, in order to maximize the conductivity of these materials, the matrix is generally constructed into a very thin film, i.e., on the order of about 25 to about 250 .mu.m. As is apparent, the reduced thickness of the film reduces the total amount of internal resistance within the electrolyte thereby minimizing losses in conductivity due to internal resistance.
The solid electrolytes also contain a solvent (plasticizer) which is typically added to the matrix primarily to enhance the conductivity of the electrolytic cell. In this regard, the solvent requirements of the solvent used in the solid electrolyte have been art recognized to be different from the solvent requirements in liquid electrolytes. For example, solid electrolytes require a lower solvent volatility as compared to the solvent volatilities permitted in liquid electrolytes.
Suitable solvents well known in the art for use in such solid electrolytes include, by way of example, propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, triglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like.
The solid, solvent-containing electrolyte has typically been formed by one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of a salt, and the electrolyte solvent (typically a glyme compound and an organic carbonate) are then added to the solution. This solution is then placed on the surface of a suitable substrate, e.g., the surface of a cathode, and the volatile solvent is removed to provide for the solid electrolyte.
In the other method, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the salt and the solvent. This mixture is then placed on the surface of a suitable substrate, e.g., the surface of the cathode, and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, and the like) so as to form the solid, solvent-containing electrolyte.
Typically, cathodes for solid electrolytic cells are prepared by providing a current collector with an adhesion promoter layer made of polyacrylic acid and carbon. A mixture of cathodic material, an electroconductive agent, solid matrix forming monomer, solvent, and viscosifying agent are then coated on the adhesion promoter layer of the current collector substrate followed by curing with e-beam or UV radiation. Alternatively, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of cathodic material, an electroconductive agent, solvent, and viscosifying agent are then are then added to the solution. This solution is then placed on the surface of the adhesion promoter layer of the current collector substrate, and the volatile solvent is removed.
When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid battery, i.e., an electrolytic cell.
Regardless of which of the above techniques is used in preparing the electrolytic cell, improvements are sought in the adhesion promoter layer. Improvements in the reduction of the acidity of the binder of the adhesion promoter layer are sought to improve the lithium cyclability of the electrolytic cell. Improvements sought in the adhesion promoter layer also include the removal of protons and the introduction of lithium ions into the carbon network, which increases the concentration of lithium. Improvements are sought which reduce the amount of protons, which can occupy intercalation sites resulting in reducing the migration of Li.sup.+ ions across the electrolyte.
In view of the above, the art is searching for methods to improve adhesion promoter layer manufacture and coatability as well as to increase the adherence of the laminate layers of solid batteries employing such layers.